La Trochita, in English known as the Old Patagonian Express, is a 750 mm narrow gauge railway in Patagonia, Argentina using steam locomotives. The nickname La Trochita means literally "The little gauge" though it is sometimes translated as "The Little Narrow Gauge" in Spanish while "trocha estrecha", "trocha angosta" in Argentina, is often used for a generic description of "narrow gauge."

750 mm narrow-gauge railways are very similar to 760 mm and 2 ft 6 in gauge. 750 mm gauge rolling stock is almost compatible with 760 and 762 mm railways.